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In a blog post, Changechecker said: "Our calculations suggest there are probably somewhere between 600,000-800,000 Edinburgh City coins still in circulation.

"On average, it means that you will have to examine roughly 3,000 mixed £1 coins to find the Edinburgh City £1 Coin."

Rare coins are much more valuable when in mint condition. And test versions, known as "proof coins" are even more valuable with collectors. These are made by The Royal Mint and are struck at a lower speed with a higher finish.

The £1 coin came into circulation in 1983 and a total of 2.2 billion have been struck.

Over the years, millions have been withdrawn because they are worn or damaged (and a few more are probably down the back of the nation's sofas). Three years ago, the Royal Mint estimated that 1,553,000,000 £1 coins are in circulation.